The present invention relates to a device for increasing the braking capacity or power of a multi-cylinder internal combustion engine of a vehicle during an engine braking operation.
The present invention starts with internal combustion engines that have an engine braking device internal to the engine as well as a single or multi-stage turbosupercharger including, for each stage, an exhaust gas turbine and a charge air compressor, and in addition are provided with two exhaust gas header pipes via which the exhaust gas respectively discharged from a plurality of cylinders, combined in groups, can be conveyed to a turbine inlet. An engine braking device is known, for example, from EP 0736672 B1 in conjunction with an engine braking process. Such a process has been successful in hundreds of thousands of internal combustion engines installed in MAN trucks and buses, and is known to those of skill in the art as the MAN-EVB (exhaust valve-brake/EVB®). With this engine braking process, during the engine braking operation the throttle valves that are disposed in the exhaust gas line are in a throttling position in which the pertaining exhaust gas line is not entirely closed, so that a portion of the retained exhaust gas can pass the throttle valve via a narrow gap that remains open at the edge. This exhaust gas leakage is necessary in order to prevent a stopping or stalling of the internal combustion engine, even until it stops running, during overheating of the engine.
Although this known engine braking process produces excellent braking power, for many applications it is desirable to have an increase in braking power during an engine braking operation in order to provide greater relief for the further braking systems, such as retarders and service brakes, that are present in the vehicle, or to make them smaller. For this purpose, auxiliary measures are necessary to enable raising of the supercharged pressure during the engine braking operation, so that significantly higher exhaust gas counter pressures are established.
Various devices and methods are already known for achieving such an increase in supercharged pressure for increasing braking power. For this purpose IVECO uses a turbosupercharger having variable turbine geometry (VTG), whereby for increasing the braking power the turbine cross-section is varied. The drawback of this is that VTG turbochargers are considerably more expensive than normal turbo super charges and due to the many small movable parts in the turbine do not achieve the surface life and efficiency required for turbochargers. Instead of a VTG charger, Daimler Chrysler, for their version known as “Turbobrake” uses a turbosupercharger with which, via a displaceable ring in the turbine of the turbocharger, the throughput thereof can be changed over from engine operation to braking operation in the sense of small throughputs and a very high pressure drop so that the charge pressure can be raised during a braking operation. However, this turbosupercharger has similar drawbacks to a VTG turbocharger. Further examples for increasing braking power can be found in EP 077 0772 B1 and EP 082 1147 B1. In both cases, upstream of a throttle valve branching off from an exhaust gas line is a bypass line that either opens out into a collection chamber within the turbine housing and from which nozzle openings proceed in the direction of the turbine wheel, or the bypass line communicates with a single nozzle bore in an intermediate wall or partition of the turbine housing that on the outlet side is disposed ahead of a nozzle ring or guide baffles via which a nozzle stream is deflected and directed in the direction of the outer periphery of the turbine wheel. These two known solutions also have the drawback that they require modifications within the turbine or special turbines, and are therefore relatively expensive.
It is therefore an object of the present invention to provide a device for increasing the braking power of a multi-cylinder internal combustion engine of a vehicle according to which in the exhaust gas turbine of the turbosupercharger, or with a dual-stage turbosupercharger in the high pressure stage thereof, complicated components and modifications can be eliminated, so that the device can be realized in a straightforward and complicated manner, yet produces a relatively high increase in braking power.